A semiconductor device fabrication process for forming a layered structure of an integrated circuit on the surface of a substrate, for example, a semiconductor wafer (hereinafter, referred to as a wafer), includes a liquid processing process for removing fine particles or a natural oxide film on the surface of the wafer by a chemical liquid and the like.
A single-wafer type spin washing apparatus, as a liquid processing apparatus used for the liquid processing process, removes particles or a natural oxide film on the surface of a wafer by rotating the wafer while supplying an alkaline or acidic liquid to the surface of the wafer by a nozzle. In this case, the liquid which remains in the surface of the wafer is removed through spin-drying by rotating the wafer after a rinsing process performed by pure water and the like.
However, as a semiconductor device is highly integrated, a problem of so-called pattern collapse has become serious in the process of removing such a liquid. The pattern collapse indicates a phenomenon where in drying of the liquid which remains in the surface of the wafer, when the liquid which remains on the left and right sides of a projection portion of a pattern forming unevenness is non-uniformly dried, the balance of leftward/rightward surface tension on the projection portion is collapsed, and thereby the projection portion is collapsed in the direction of the liquid which remains in a larger amount.
As a method for removing the liquid which remains on the surface of a wafer and inhibiting such pattern collapse, there has been known a drying method using a supercritical-state fluid (supercritical fluid). A supercritical fluid has a lower viscosity, compared to a liquid, and also has a high liquid-solubility. Besides, in the supercritical fluid, there exists no interface between liquid-gas. Accordingly, when a wafer with a liquid remained thereon comes in contact with the supercritical fluid, and the liquid on a wafer surface is dissolved in the supercritical fluid, it is possible to dry the liquid without an influence of surface tension.
Herein, the supercritical state requires a high temperature and high pressure condition. The inventors of the present disclosure have examined a method for performing a liquid drying processing by using a supercritical fluid (hereinafter, referred to as supercritical drying). In the method, after washing of a wafer by a liquid, the wafer with the liquid remained thereon is loaded in a processing chamber, a fluid (in a liquid state at room temperature) is supplied into the processing chamber, the processing chamber is tightly closed, and then a supercritical fluid is obtained by heating the fluid.
As shown in FIG. 1 and paragraph [0033] of Japanese Laid-Open Patent Publication HEI. No. 9-232271, the fluid supplied into the processing chamber is heated by a heater (such as a heating resistor) to be placed in a supercritical state, in which the heater is provided in a seating unit on which a wafer is seated. However, during the loading of the wafer, when the temperature of the atmosphere within the processing chamber or the region where the wafer is disposed is high, the liquid is naturally dried, and thereby pattern collapse occurs before the start of supercritical drying.
Therefore, when the wafer is loaded into the processing chamber, the temperature of the atmosphere within the processing chamber or the region where the wafer is disposed is required to be lowered. However, if the heater is turned off in a period of time other than the period of time for heating a fluid, the fluid is required to be heated again. Thus, a time required for obtaining a supercritical fluid is prolonged, thereby reducing the throughput of a device. Also, the on/off conversion of the heater according to the performance of each supercritical drying process facilitates the deterioration of a switch of the heater or the heater itself, thereby causing a problem in that a maintenance interval is shortened.